Conventional optical waveguides use one of three methods for confinement of a signal: 1) specular reflection; 2) total internal reflection; and 3) transverse Bragg reflection. As shown in FIG. 1, a metallic waveguide 100 consisting of a first mirror 120 and a second mirror 130 employs specular reflection at the waveguide boundaries in order to confine an optical signal 101 to the waveguide region. In reflection from the metallic surface, the field oscillations of the signal, such as, for example, an electromagnetic plane wave incident on a metallic surface at angle θi with respect to the surface normal (shown as the dotted line), act to drive conduction electrons such that they radiate. In accordance with Maxwell's equations and the boundary conditions, the form of this radiation is again a plane wave with angle θr with respect to the surface normal such that θi equals θr. For long wavelength radiation, such as microwaves, this process occurs with very low losses making metallic waveguides practical. At optical frequencies, the intrinsic loss from a single reflection becomes non-negligible, so that metallic optical frequency waveguides are very lossy and not of practical importance.
Total internal reflection is inherently a low loss process regardless of frequency, and therefore is a common confinement mechanism used at optical frequencies. FIG. 2 depicts a slab waveguide 200 including a dielectric core 210, a first dielectric cladding 220, and a second dielectric cladding 230. In accordance with Snell's law, the tangential component of momentum is conserved when a signal 201, such as, for example, a plane wave crosses an interface between dielectrics. For incident angles beyond the critical angle, the transmitted wave cannot satisfy conservation of momentum, and therefore is not allowed. Conservation of energy is maintained by a complete transfer of incident energy to the reflected wave in the high index medium. Equating tangential components of the wave vector momentum for the incident and reflected waves, results in θr, the angle of reflection with respect to the interface normal, being equal to θi just as in reflection from a metallic surface.
The third confinement mechanism uses transverse Bragg reflection as the confinement mechanism for a dielectric waveguide. FIG. 3 depicts a conventional transverse Bragg waveguide 300 including a dielectric core 310, a first cladding 320, and a second cladding 330. First cladding 320 and second cladding 330 each consist of periodic layers with alternating high and low dielectric constants. The direction of periodicity (parallel to the x-axis) of the dielectric constant (or index of refraction) is normal to the optical axis (parallel to the y-axis) of waveguide 300. The nature of Bragg reflection is such that a signal 301, such as, for example, a plane wave has an incident angle θi similar to a reflected angle θr relative to the normal to the Bragg layers responsible for the reflection. In the conventional transverse Bragg waveguide shown in FIG. 3, this implies that, in terms of confinement, the momentum transfer resulting from the Bragg scattering process is oriented normal to the waveguide axis, and θi equals θr as in the metallic and conventional dielectric waveguides discussed above.
Conventional dielectric slab waveguides, metallic parallel plate waveguides, and transverse Bragg waveguides exhibit inversion symmetry. Inversion symmetry implies that a mode reflected at an edge boundary perpendicular to the end of the waveguide is also guided. The result is that at a planar boundary perpendicular to the waveguide axis, any reflected energy is guided down the waveguide in the reverse direction. In certain applications, guiding of this reflected energy is problematic. For example, in high-gain semiconductor-optical amplifiers, these reflections constitute undesirable feedback, which can result in spurious lasing. Similarly, in fiber lasers, such guided reflections from stimulated Raman and Brillouin scattering can limit the power output of these devices and otherwise impact the spectral quality of the laser.
Thus, there is a need to overcome these and other problems of the prior art to provide optical waveguides that do not exhibit inversion symmetry.